Investigation of shallow to deep subsoil structures in urban area by means of microtremors and its application to an assessment for regional seismic input motions.

本研究的目的是通过实验的方式发展一种可行的方法来评估城市地区的地震输入运动，并选择了北方日本的札幌城市地区作为试验场。首先，观测系统在硬件和软件两方面都在升级。在笔记本电脑中安装A/D转换器，通过计算机程序实现对温度的真实的实时监测。在忙碌繁忙的商业区白天测量，尽管S/N比低，该系统可以进行。在札幌市区进行了广泛的测量，在1990年和1991年的长周期脉动。设置了7条平行于海岸线的观测线和2条垂直于海岸线的观测线，覆盖面积约40 × 40公里，共获得183个站点的记录。在北海道大学设置了一个参考点，以监测测量期间的脉动时间变化，通过将每个点的傅立叶谱除以参考点的相应谱，得到每个点的谱比。将这些比值按0.5、1、2和4秒的周期分别绘制在图上，并绘制了四个等值线图作为地震小区划图。这些图用1982年浦川近海地震的各种少量深钻资料、三次强震记录和地震烈度分布资料进行了验证。

The purpose of this study is to develop a feasible method for evaluating seismic input motions in an urban area by means of experimental way and Sapporo urban area, northern Japan, was selected as a test field.Microtremors measurement was adapted for this study because of its easy applicability for detecting subsoil conditions. At the first, the observation system was upgrading in both terms of hardware and software. An A/D converter was installed in a laptop computer and a real time monitoring was introduced by computer program. With this system daytime measurements in busy business district can be performed in spite of low S/N ratio.Extensive measurements of long-period microtremors were conducted in Sapporo urban area in 1990 and 1991. Seven observation lines parallel to the coastal line and two lines perpendicular to these were set up, covering the area of about 40 x 40 kilometers, obtaining records at totally 183 sites. One reference site was set up at Hokkaido University to monitor the time variation of microtremors during the period of measurements.Spectral ratios were obtained for each site by dividing the Fourier spectra by the corresponding spectra at the reference site. These ratios were plotted on a map for each period of 0.5, 1, 2 and 4 second and four contour maps were drawn as seismic microzonig maps. These maps were verified by various of small number of deep drilling data, three strong motion records and seismic intensity distribution data of the 1982 Off-Urakawa earthquake.